Download 7.Mesoderm Derivatives

MESODERMAL DERIVATIVES
By: Dr. Mujahid Khan
Derivatives
 Connective
tissue
 Cartilage
 Bone
 Striated
& smooth muscles
 Heart
 Blood
& lymphatic vessels
 Kidneys, ovaries, testes & genital ducts
 Serous membrane lining the body cavities
 Spleen & cortex of the supra renal gland
Development of Somites

As the notochord and neural tube forms

Embryonic mesoderm on each side of them
proliferates

Form a thick longitudinal columns of paraxial
mesoderm

Each column is continuous with intermediate
mesoderm
Development of Somites
 Intermediate
mesoderm gradually thins
into a layer of lateral mesoderm
 Lateral
mesoderm is continuous with the
extraembryonic mesoderm
 Extraembryonic
sac and amnion
mesoderm covers the yolk
Somites
 Paraxial
mesoderm differentiates and
begins to divide into cuboidal bodies called
somites by the end of 3rd week
 These
blocks of mesoderm are located on
each side of developing neural tube
 About
38 pairs of somites form during the
somite period of human development (2030 days)
Somites

About 42-44 pairs of somites are present by the
end of 5th week

Are triangular in transverse section

Form distinct surface elevations on the embryo

Are used as one of the criteria to know the age
of the embryo at this stage
Somites

First appear in the future occipital region

Soon develop craniocaudally

Gives rise to the axial skeleton and associated
musculature

Also forms adjacent dermis of the skin

The first pair of somites appear at the end of 3rd
week
Somites
 First
appear at a short distance caudal to
the cranial end of the notochord
 Subsequent
sequence
pairs form in a craniocaudal
Intraembryonic Coelom
 Also
known as primordium of embryonic
body cavity
 Appears
as isolated coelomic spaces in
the lateral mesoderm and cardiogenic
mesoderm
 These
spaces soon coalesce to form a
single horseshoe shaped cavity called
intraembryonic coelom
Parietal & Visceral Layers
 Somatic
or parietal layer continuous with
the extraembryonic mesoderm covering
the amnion
 Splanchnic
or visceral layer continuous
with the extraembryonic mesoderm
covering the yolk sac
Parietal & Visceral Layers
 Somatic
mesoderm with overlying
embryonic ectoderm form the embryonic
body wall or somatopleure
 Splanchnic
mesoderm with underlying
embryonic endoderm form the embryonic
gut or splanchnopleure
Fate of Intraembryonic Coelom
During the 2nd month, the intraembryonic
coelom is divided into 3 body cavities:
 Pericardial
 Pleural
cavity
cavity
 Peritoneal
cavity
Early Development of
Cardiovascular System
 Starts
at the beginning of the 3rd week
 Vasculogenesis
and angiogenesis begins
in the extraembryonic mesoderm of the
yolk sac, connecting stalk and chorion
 Embryonic
blood vessels begin to develop
about 2 days later
Early Development of
Cardiovascular System
 The
urgent need for blood vessels to bring
nourishment and oxygen to the embryo
from mother causes the early formation of
the cardiovascular system
 A primordial
uteroplacental circulation
develops during the 3rd week
 Until
then the embryonic nutrition is
obtained from maternal blood by diffusion
Vasculogenesis & angiogenesis
Formation of embryonic vascular system
involves 2 processes:
 Vasculogenesis
 Angiogenesis
Vasculogenesis

Mesenchymal cells differentiate into endothelial
precursors called Angioblast

Angioblast aggregate to form isolated
angiogenic cell clusters or blood islands

Small cavities appear within the blood islands

Angioblasts flatten to form endothelial cells
Vasculogenesis
 Endothelial
cells arrange themselves
around the cavities in blood island to form
the endothelium
 These
endothelium lined cavities soon
fuse to form networks of endothelial
channels called Vasculogenesis
Angiogenesis
 Vessels
sprout into adjacent areas by
endothelial budding and fuse with other
vessels called Angiogenesis
Development of Blood Cells

Blood cells develop from the endothelial cells of
vessels called hemangioblasts

Develop at the end of 3rd week on the yolk sac
and allantois

Hematogenesis does not begin until 5th week

It occurs first in liver and later in spleen, bone
marrow & lymph nodes
Development of Blood Cells
 Fetal
and adult erythrocytes are derived
from different hematopoietic progenitor
cells (hemangioblasts)
 Mesenchymal
cells surrounding the
primordial endothelial blood vessels
differentiate into the muscular and
connective tissue elements of the vessels
Primordial Cardiovascular System
 Heart
& great vessels develop from
mesenchymal cells in the cardiogenic area
 Paired
longitudinal endothelial lined
channels or endocardial heart tubes
develop during the 3rd week
 These
tubes fuse to form the heart tube
Primordial Cardiovascular System
 The
tubular heart joins with blood vessels
in the embryo, connecting stalk, chorion
and yolk sac to form a primordial
cardiovascular system
 Heart
begins to beat on 21-22 days and
blood circulates
 CVS
is the first organ system to reach a
functional state
Further Development of
Chorionic Villi
 Primary
chorionic villi becomes secondary
chorionic villi as they acquire
mesenchymal cores
 Before
the end of third week capillaries
develop in the secondary chorionic villi
 Now
it is called tertiary chorionic villi
Further Development of
Chorionic Villi

Cytotrophoblastic extensions from these stem
villi join to form a cytotrophoblastic shell that
anchors the chorionic sac to the endometrium

The rapid development of chorionic villi during
the third week greatly increases the surface area
of chorion

This causes exchange of oxygen and nutrients
between the maternal and embryonic
circulations